HCV-related liver disease is a leading cause of death among HIV co-infected patients;nonetheless, HCV treatment uptake rates remain very low in this population, especially among drug users. Little is known about the barriers to HCV treatment among co-infected patients with current or past drug dependency and the factors that contribute to treatment decisions on the part of either patients or providers. This exploratory study will use a mixed-methods design and a process model framework to examine HCV treatment decisionmaking in the context of HIV/HCV co-infection. We propose to use both qualitative and quantitative methods to explore factors related to the provider, patient, and clinic organizational structure that influence HCV treatment decisions. The study will be conducted at 3 HIV clinics in the LA that vary on factors that may influence HCV treatment decisionmaking, such as number of patients, involvement of liver specialists in HCV care, and current HCV treatment rates. First, qualitative interviews will be conducted with HCV providers, co-infected patients with past or current drug dependency, and clinic administrators and other stakeholders to explore the factors and process by which HCV treatment decisions are made. Results from these interviews will inform the development and refinement of measures and hypotheses for use in quantitative surveys of both patients and HCV providers and chart abstraction to determine the rate at which co-infected drug users are offered and prescribed HCV treatment, identify factors that influence treatment decisions, and characterize patients for whom treatment is actually offered and prescribed, as well as for whom treatment is deferred. Study results will describe HCV treatment rates for patients with histories of drug dependency at a major U.S. epicenter of HIV/HCV co-infection, begin to illuminate a multilevel model of factors contributing to HCV treatment decisions, and may inform the development of an intervention (and future grant application) to improve HCV treatment uptake and outcomes among this population. This study is important to public health as its findings may guide the development of interventions and policy efforts targeting various stakeholders (provider, patient, HMOs) in the goal of improving HCV care for HIV patients with drug dependency histories, as well as broadening the understanding of treatment decisionmaking in the context of complex, chronic comorbid diseases.